1. Field of the Invention
The present invention is related to the use of shock struts, and more particularly the use of shock struts having two cambers, or stages, of dampening characteristics.
2. Description of Related Art
Generally, shock struts are used to cushion or dampen loads on various types of vehicles and machinery. Beyond the various load characteristics that must be considered when selecting a shock strut, practical concerns such as available space and weight must also be considered. Shock struts used for aircraft typify such constraints. While the aircraft is in flight, the shock strut and the landing wheel carried by it are usually retracted into the aircraft fuselage or wing. The presence of the well imposes restrictions on the location of structural members, fuel tanks and other components. Therefore, it is desirable to minimize the size of the landing gear. Further, the height of the landing gear is affected by the length of its shock strut and the amount of retraction or projection of the shock strut during loading. The height of the landing gear, in turn, affects the xe2x80x9csill heightxe2x80x9d of the airplane, which is the height of the door opening on the fuselage when the plane is resting on the tarmac. For very large planes taller landing gear places the sill height out of reach of much of the ground equipment designed to service smaller airplanes.
U.S. Pat. No. 3,888,436 to Sealey (xe2x80x9cSealeyxe2x80x9d) discloses an airplane landing gear. The landing gear includes an inner cylinder 11 and an outer cylinder 10, as shown in FIG. 1 of Sealey. During compression, the inner cylinder moves within the outer cylinder and causes fluid from a Chamber F to be forced between a metering pin 29 and an orifice lip 21, as shown in FIG. 2 of Sealey. The fluid flows upward inside a tube 16 and into Chamber A through ports 22 so as to compress the gas in Chamber A. Increased pressure in Chamber A, in turn, drives fluid from Chamber A through ports 24 to enter Chamber G. Because the flow between the metering pin and the lip is constricted, the pressure in Chamber F rises rapidly as the strut is initially compressed and provides a hard spring for the large forces that occur during landing.
After initial compression, the pressure in Chamber D gradually increases as liquid is metered into it through taxi bleed ports 35. A piston 32 defining one end of Chamber D disengages a bleed port cylinder 30 at the other end of Chamber D and the fluid flows with greater freedom from Chamber F into Chamber D, as shown in FIG. 4 of Sealey. Also, as the piston moves downward it compresses a gas in Chamber C. In this manner the two gas volumes in Chambers A and C are in operation and provide a relatively soft spring for the lower forces that occur during taxiing. Despite the advantages of the landing gear disclosed by Sealey, additional adaptations and improvements in shock-strut design are desirable to fit newer aircraft and airport standards.
It would be advantageous to have a shock strut that provides two-phases of support, one for touchdown upon landing and another for taxiing of the aircraft. In addition, it would be advantageous if the shock strut were to have a compact construction so as to allow its use on planes having sill height restrictions.
The present invention addresses the above needs and achieves other advantages by providing a shock strut for supporting and dampening a load with a two-phase dampening characteristic. The shock strut includes an inner cylinder slidably mounted within an outer cylinder, each of the cylinders having slidably mounted therein a piston. A first one of the pistons in the outer cylinder defines a first gas chamber, a second one of the pistons in the inner cylinder defines a second gas chamber. In between the cylinders is a metering device that defines a third and fourth fluid chambers between it and the first and second pistons, respectively. The metering device controls fluid flow between the third and fourth fluid chambers. At full extension of the shock strut, a first stage of dampening is provided by the first gas chamber is compressed by fluid flowing into the third chamber from the fourth chamber. As the shock strut is compressed, a second stage of dampening is provided by fluid flowing into the fourth chamber and compressing the second gas chamber.
A shock strut of one embodiment of the present invention for supporting and dampening a load with a two-phase dampening characteristic includes an outer cylinder, an inner cylinder, first and second pistons and a fluid metering device. Each of the outer and inner cylinders have closed and open ends. The inner cylinder is slidably mounted within the open end of the outer cylinder. Slidably mounted between the ends of the outer cylinder is the first piston, which defines a first fluid chamber between it and the closed end of the outer cylinder. Slidably mounted between the ends of the inner cylinder is the second piston, which defines a second fluid chamber between it an the closed end of the outer cylinder. Positioned between the pistons is a fluid metering device, wherein a third fluid chamber is defined between the metering device and the first piston. The third fluid chamber is on an opposite side of the first piston from the first fluid chamber. The metering device also defines a fourth fluid chamber between it and the second piston, on an opposite side of the second piston from the second fluid chamber. The metering apparatus progressively controls fluid flow into the third and fourth chambers, leading to compression of the first and second fluid chambers. Compression of the first and second fluid chambers results in the two-phase dampening characteristic.
In one aspect, the first and second fluid chamber contain a fixed amount of a gas and the third and fourth chambers contain a relatively incompressible fluid, such as a hydraulic fluid.
In another aspect, the fluid metering apparatus includes a metering pin extending through an orifice defined by a metering plate. The metering pin defines an elongate opening having sufficient length to maintain fluid communication between the third and fourth fluid chambers from full extension through full compression of the shock strut. A clearance defined between the metering pin and plate is configured to control fluid flow from a fifth chamber into the fourth chamber, causing compression of the second piston and the second fluid chamber. The elongate opening of the metering pin may also include a neck on an end adjacent the first fluid chamber to further control fluid flow.
In still another aspect, the outer cylinder of the shock strut may include a housing for supporting therein a first piston-supporting wall structure. The first piston-supporting wall structure is configured to slidably support the first piston. In addition, the inner cylinder may include its own housing for supporting therein a second piston-supporting wall structure configured to slidably support the second piston. A metering pin of the metering device is supported on an open end of the first piston-supporting wall structure. A metering plate of the metering device may be supported on an open end of the second piston-supporting wall structure. Extending through the metering plate is the metering pin.
Optionally, the metering device may include a fifth fluid chamber in fluid communication with the third fluid chamber through at least one orifice defined in the first piston-supporting wall. In addition, fluid communication is established between the fifth fluid chamber and the fourth fluid chamber through a clearance defined between the metering pin and the metering plate.
The shock-strut of the present invention has several advantages. It has two-stage loading characteristics that provide sufficient stiffness to resist high landing forces in the first stage, and increased stiffness for a less bouncy taxiing ride. The arrangement of the fluid chambers provides the second stage loading characteristics at a height sufficiently low to allow docking of the aircraft at standard airport gates and access by standard airport equipment. This also allows the extended shock strut pressure to be low enough to lessen the impact load at touchdown. The compact profile also allows the shock-strut to be retrofit into pre-existing aircraft configured for single-stage, conventional shock-struts. This design will allow a single shock strut to be used without replacing the inner and outer cylinders. The use of the metering pin and the metering plate provides a robust, all mechanical fluid metering device for use in an aircraft.